The present invention relates to a display device and a liquid crystal display device, particularly, to a display device and a liquid crystal display device that perform color display by emitting light from a light source through a color filter layer.
In recent years, liquid crystal display devices are utilized in a wide range of fields from medium-sized and large-sized displays used for computers, television sets and the like to small-sized displays used for car navigation systems and mobile telephones. A liquid crystal display device includes a backlight unit and a liquid crystal display panel. The liquid crystal display device performs image display by the liquid crystal display panel that controls transmission of light from the backlight unit.
Among them, active matrix liquid crystal display devices, which use active elements such as thin film transistors (TFTs) and metal-insulator-metal (MIM), are drawing attention because of excellent display characteristics. An active matrix liquid crystal display device normally includes a TFT array substrate having TFTs as active elements being arrayed in a matrix and an opposing substrate that opposes to the TFT array substrate, and liquid crystal is filled between the two substrates.
The liquid crystal display device includes a display region composed of a plurality of pixels, each of which having a display electrode and a TFT. Light transmittance is varied by application of an electric field to the liquid crystal with the display electrode, thus performing image display. In a color liquid crystal display device, a color filter layer for performing color display is normally provided on the opposing substrate. The color filter layer is composed of three layers of red (R), green (G) and blue (B) side by side, and a black matrix layer formed between these respective color filter layers. Each of the color filter layers transmits light of only a specified range of wavelengths, thus displaying a desired color. Each of the pixels performs color display of any one of R, G and B, whereby an entire display screen can display desired color images.
Regarding a color display device including the liquid crystal display device, two important factors are known from the view point of display quality. One is luminance of the display device, and the other is a reproducible range of colors. In order to perform high-definition display, high luminance and a wide reproducible range of colors are required. Particularly, in light of the reproducible range of colors, such color reproducibility is required to approximate assumed primary color coordinates of a National Television System Committee (NTSC) color TV system as closely as possible. That is, it has been deemed ideal to bring an NTSC ratio to 100%. Here, the NTSC ratio refers to an a real ratio of a triangle of a color reproduction region realized by a display device with respect to an area of a triangle formed by a color reproduction region of NTSC in a chromaticity coordinate system.
Nevertheless, it has been deemed impossible to achieve color reproducibility at an NTSC ratio of 100% with a conventional liquid crystal display device. It is attributed to the fact that widening the range of color reproducibility requires either considerable thickening of the color filter or considerable condensing of photosensitive pigments contained in the color filter layer. Thickening a film of the color filter layer incurs two problems. One of the problems is that optical transmittance is largely reduced by thickening the film of the color filter layer (or by condensing the pigments), and thus sufficient luminance cannot be secured. For this reason, in liquid crystal display devices used in general, an NTSC ratio of an LCD used for a note PC has been limited to about 45%, and an NTSC ratio of a stationary-type liquid crystal display monitor has been limited to about 70%.
Moreover, in order to achieve the NTSC ratio of 100% with a conventional fluorescent light tube and a color filter layer, experiments proved that a color filter layer required a thickness of about 8 micrometers. Such thickness outsteps a boundary of practically produceable color filter layers. Of course, the thickness can be reduced if a pigment density is increased. However, the pigment density also has a certain limitation attributable to curing of a base member (acrylic resin and the like) of the color filter layer. Furthermore, increasing energy supplies to a lamp may raise luminance of the lamp, however, such energy supplies are also limited because of problems concerning heat generation, durability of electrodes and the like.
Accordingly, it has been conceived that fluorescent light tubes with higher luminous efficiency were necessary for achieving a high NTSC ratio and securing sufficient luminance at the same time.
FIG. 10 is a graph showing a radiant energy spectrum from a cold cathode fluorescent light tube and spectral transmittance of a color filter layer in a conventional backlight unit. The graph in the drawing corresponds to a conventional liquid crystal display device of an NTSC ratio of 70%. In FIG. 10, the x axis indicates a wavelength of light. The y axis on the left corresponds to a radiant energy spectrum of a lamp, and its unit is an arbitrary unit. The y axis on the right indicates transmittance of a color filter layer. In the drawing, reference numerals 1001, 1002 and 1003 respectively denote spectral transmittance of a blue filter layer, spectral transmittance of a green filter layer and spectral transmittance of a red filter layer.
In the conventional liquid crystal display device, a tri-phospher fluorescent light tube is used as a light source of the backlight unit. The inside of the fluorescent light tube is coated with three kinds of phosphors, each of which emits light corresponding to RGB, respectively. The backlight unit obtains the light from the light source (the lamp) by allowing the phosphors to emit light. The phosphor s conventionally used are as follows: BaMg2Al16O27:Eu for a blue phosphor; LaPO4:Ce,Tb for a green phosphor; and Y2O3:Eu for a red phosphor, and the like.
FIG. 10 shows radiant energy spectra of the three kinds of the phosphors, namely, BaMg2Al16O27:Eu, LaPO4:Ce,Tb and Y2O3:Eu. In FIG. 10, reference numerals 1004, 1005 and 1006 are a radiant energy spectrum of the blue phosphor, a radiant energy spectrum of the green phosphor and a radiant energy spectrum of the red phosphor, respectively. The blue phosphor possesses a maximum peak of the spectrum in the vicinity of 450 nm. The peaks near 405 nm and 435 nm indicate light emission of Hg filled in the fluorescent light tube. The green phosphor possesses a maximum peak of the spectrum in the vicinity of 545 nm and sub peaks respectively in the vicinity of 490 nm, 590 nm and 620 nm. Note that, light emission of Hg is also observed in the vicinity of 580 nm. The red phosphor possesses a maximum peak of the spectrum in the vicinity of 610 nm.
The inventors of the present invention paid attention in particular to the radiant energy spectrum of the conventional green phosphor. The green phosphor possesses two sub peaks apart from the maximum peak. And the sub peak on the short-wave side is located approximately in the middle of a wavelength region where spectral transmittance curves of the blue color filter layer and the green color filter layer overlap. In addition, the sub peak on the long-wave side is located approximately in the middle of a wavelength region where spectral transmittance curves of the green color filter layer and the red color filter layer overlap.
Each of the light at the sub peaks is recognized as a major factor obstructive to color purity of the liquid crystal display device, because the light at the sub peaks is intensely emitted from both of the blue color filter layer and the green color filter layer, or from both of the green color filter layer and the red color filter layer.
Therefore, it is conceivable that a liquid crystal display device of a high NTSC ratio and of high luminance can be obtained if a high-luminance green phosphor without the above-described sub peaks becomes usable. However, such a phosphor higher in luminous efficiency than the conventional green phosphor without light emission corresponding to the above-described sub peaks has not been found to date.
Accordingly, the inventors paid further attention to a relation between transmittance and a film thickness (or a quantity of a photosensitive material) of the color filter layer. The transmittance of the color filter layer decreases exponentially with respect to its thickness. That is, when transmittance of a color filter layer having a thickness of X is Y %, then transmittance of a color filter layer having a thickness of 2xc3x97 is Y2%. Therefore, an attrition ratio of the luminance becomes larger as the thickness of the color filter layer is increased. The above-described phenomenon is also applicable to density or a quantity of the photosensitive material.
FIG. 11 is a graph showing a state how the luminance of the light transmitted through the color filter layer decreases in the case that the thickness of the color filter layer is increased for gaining a larger NTSC ratio in the conventional color filter layer and the cold cathode fluorescent light tube as the light source. In FIG. 11, the y axis indicates a luminance ratio and its unit is an arbitrary unit. The x axis thereof indicates an NTSC ratio of a color reproduction region. As can be understood from the drawing, the luminance is decreased as the NTSC ratio is increased, and in particular, an attrition ratio of the luminance becomes considerably large in a range where the NTSC ratio is about 85% or higher, thus sharply decreasing the luminance.
In other words, the inventors have discovered that the luminance of the light transmitted through the color filter layer can be made larger than the conventional liquid crystal display device of the same NTSC ratio, if a thickness of the color filter layer can be made sufficiently thinner than a conventional thickness in spite of small luminous efficiency as a cold cathode tube.
Therefore, one object of the present invention is to provide a color display device capable of securing sufficient luminance while achieving a high NTSC ratio.
As a result of devoted research in order to achieve a liquid crystal display device capable of securing sufficient luminance with a high NTSC ratio, particularly one which the NTSC ratio thereof is about 100% which has been previously deemed unpracticable, the inventors have found out the following points.
In order to secure sufficient luminance of a display device in a high NTSC ratio, balancing of radiant energy spectrum distribution of a fluorescent light tube, luminous efficiency thereof and factors of transmittance characteristics of a color filter layer is extremely important, instead of enhancing the luminous efficiency of the fluorescent light tube itself. In other words, the inventors have found out for the first time that luminance larger than that of the conventional liquid crystal display device can be obtained in a region of high NTSC ratios of a display device, even in the case of a fluorescent light tube that is relatively inferior in its luminous efficiency to a conventional fluorescent light tube.
A first aspect of the present invention is a liquid crystal display device including a fluorescent light tube as a light source and a liquid crystal display panel for displaying images by controlling transmission of light from the fluorescent light tube. The liquid crystal display panel includes: a color filter substrate having color filter layers of red, green and blue; an opposing substrate that opposes to the color filter substrate; and a liquid crystal material being filled between the opposing substrate and the color filter substrate. The fluorescent light tube includes a phosphor having 80% or lower luminous efficiency in comparison with LaPO4:Ce,Tb, as a green phosphor. A maximum peak of a radiant energy spectrum of the phosphor is included within a spectral transmissive region of the green color filter layer. In addition, concerning points other than the maximum peak, the radiant energy spectrum of the phosphor increases virtually continuously in a wavelength region where spectral transmissive regions of the blue and the green color filter layers overlap. The fluorescent light tube and the color filter layers have a relation in that a color reproduction region of the light emitted from the fluorescent light tube through the color filter layers has an NTSC ratio of 85% or higher.
It is preferable that the radiant energy spectrum of the phosphor decreases virtually continuously in a wavelength region where the spectral transmissive regions of the green color filter layer and of the red color filter layer overlap, concerning the points other than the maximum peak. Moreover, it is preferable that a wavelength of the maximum peak of the radiant energy spectrum of the green phosphor is included within a wavelength region having 90% or higher transmittance of the maximum transmittance of the green color filter layer. Alternatively, it is preferable that the maximum transmittance of the green color filter layer is 55% or higher and the maximum transmittance of the blue color filter layer is 40% or higher.
A second aspect of the present invention is a liquid crystal display device including a backlight unit and a liquid crystal display panel for displaying images by controlling transmission of light from the backlight unit. The liquid crystal display panel includes: a color filter substrate having color filter layers of red, green and blue; an opposing substrate that opposes to the color filter substrate; and a liquid crystal material being filled between the opposing substrate and the color filter substrate. The backlight unit includes a plurality of cold cathode tubes, the plurality of cold cathode tubes being disposed in the back of the liquid crystal display panel and having any one of Zn2SiO4:Mn and 3(Ba,Mg,Eu,Mn)0.8Al2O3 as a green phosphor. The liquid crystal display device further includes a diffusion plate that is disposed between the plurality of cold cathode tubes and the liquid crystal display panel and diffuses the light from the cold cathode tubes.
Preferably, the plurality of cold cathode tubes and the color filter layers have a relation in that a color reproduction region of the light emitted from the plurality of cold cathode tubes through the color filter layers has an NTSC ratio of 85% or higher.
A third aspect of the present invention is a liquid crystal display device including a fluorescent light tube as a light source and a liquid crystal display panel for displaying images by controlling transmission of light from the fluorescent light tube. The liquid crystal display panel includes: a color filter substrate having color filter layers of red, green and blue; an opposing substrate that opposes to the color filter substrate; and a liquid crystal material being filled between the opposing substrate and the color filter substrate. The fluorescent light tube includes a phosphor having 80% or lower luminous efficiency in comparison with LaPO4:Ce,Tb, as a green phosphor. A maximum peak of a radiant energy spectrum of the green phosphor is included within a spectral transmissive region of the green color filter layer, and the radiant energy spectrum of the green phosphor has a value not exceeding 20% of a maximum peak value of a radiant energy spectrum of a blue phosphor coated inside the fluorescent light tube, at a wavelength where spectral transmittance curves of the blue and the green color filter layers intersect. The fluorescent light tube and the color filter layers have a relation in that a color reproduction region of the light emitted from the fluorescent light tube through the color filter layers has an NTSC ratio of 85% or higher.
A fourth aspect of the present invention is a display device including a tri-phospher fluorescent light tube, optical elements for controlling transmission of light from the tri-phospher fluorescent light tube, and a substrate having color filter layers of red, green and blue. The tri-phospher fluorescent light tube has three kinds of phosphors, each of which radiates any one of blue, green and red light, respectively, and has luminous efficiency not exceeding 90% of a tri-phospher fluorescent light tube having BaMg2Al16O27:Eu, LaPO4:Ce,Tb and Y2O3:Eu as phosphors. In addition, radiant energy of the tri-phospher fluorescent light tube at a wavelength where spectral transmittance curves of the blue and the green color filter layers intersect is 50% of a maximum peak of radiant energy of the blue phosphor or less. The tri-phospher fluorescent light tube and the color filter layers have a relation in that a color reproduction region of the light emitted from the tri-phospher fluorescent light tube through the color filter layers has an NTSC ratio of 85% or higher.
A fifth aspect of the present invention is a display device including: a fluorescent light tube having any one of Zn2SiO4:Mn and 3(Ba,Mg,Eu,Mn)0.8Al2O3 as a phosphor; optical elements for controlling transmission of light from the fluorescent light tube; and a substrate having color filter layers of red, green and blue. The fluorescent light tube and the color filter layers have a relation in that a color reproduction region of the light emitted from the fluorescent light tube through the color filter layers has an NTSC ratio of 85% or higher.
Preferably, a color reproduction region of the light emitted through the color filter layers has an NTSC ratio of 100% or higher. Preferably, a wavelength of a maximum peak of a radiant energy spectrum of the green phosphor is included within a wavelength region having 90% or higher transmittance of the maximum transmittance of the green color filter layer. Alternatively, it is preferable that the maximum transmittance of the light of the green color filter layer is 55% or higher, and the maximum transmittance of the light of the blue color filter layer is 40% or higher. The display device may possibly take the form of a liquid crystal display device. The liquid crystal display device includes a first transparent substrate, a second transparent substrate, and a liquid crystal display panel having a liquid crystal material filled between the first and the second transparent substrates as the optical elements. The liquid crystal display panel includes a color filter layer and a plurality of pixel electrodes being transparent electrodes arranged in a matrix, for applying electric fields to the liquid crystal material.